This invention relates generally to brake systems for tractor-trailer type vehicles and in particular a tractor brake system in which the front-to-back braking ratio is automatically modified when the tractor is operating without a trailer.
Present day tractor-trailer brake systems are generally pneumatically operated. The wheel brakes are energized by actuators, one of which is mounted at or near each wheel. The flow of pressure to these actuators is controlled by an operator actuated treadle valve. In some systems, the output pressure of the treadle valve is conveyed directly to the actuators, whereas in other systems, the treadle valve merely generates a signal pressure which is conveyed to a relay valve which in turn controls or modulates the flow of a reservoir pressure to the brake actuators.
Optimum braking rates for the various wheel assemblies will be dependent on the operating mode of the vehicle. In a heavily loaded vehicle, the trailer brakes and rear tractor brakes should be responsible for a greater portion of the braking. This is necessary to effect controlled braking and to reduce the possibility of premature wheel lockup.
When the tractor is operated without a trailer or, using industry parlance, when the tractor is operated "bobtail", the premature wheel lockup of the rear tractor wheels is of concern. The lockup of the rear tractor wheels can result in wheel hop and even lateral skidding of the vehicle.
Wheel hop generally increases the braking distance of the tractor. The loss of adhesion between vehicle tire and road surface is the most obvious cause of the extended braking distance. It has been reported that in some instances, the operator's reflex action to the wheel hop, is the removal or reduction of brake pedal foot pressure. This operator action, substantially increases the braking distance for it reduces the braking contribution of the front wheels as well as the hopping rear wheels.
One proposed method of solving this problem is the incorporation of anti-skid devices on the various wheel assemblies. These anti-skid devices generally include electronics which monitor wheel speed prior to and during braking and reduce pressure to the brake actuators should wheel lockup be imminent. At least a portion of the electronics used in these systems is generally mounted in or near the wheel assemblies. It should be readily appaent that as a result of their mounting location they are exposed to adverse environments including temperature extremes, rain, snow, road salt, and solvents used to clean vehicles for service. Additionally, they are exposed to vibration when the vehicles are in use. These factors result in frequent servicing being required not only in the electronics, but also in the interconnections and wiring among the various components.
Anti-skid devices, like anything which is electronic or mechanical, can fail. Failure of an anti-skid system may not be manifested until the vehicle operator encounters a severe braking situation. The failure during severe braking may result in unexpected wheel lockup and therefore an undesirable situation.
Because these systems generally involve sophisticated electronics to process and interpret the signals being generated by the sensors, they can significantly increase the initial cost of the vehicle. Repair of these devices is not only expensive, but should only be done by properly trained technicians. It may be difficult to obtain a system repair when on the road and down time for the repair can be excessive.
In summation electronic anti-skid devices are disliked by many vehicle operators because, in the view of these operators, these devices detract significantly from the profitability of a trucking operation. It is not just the original cost and the cost of service, but more importantly, the substantial loss of operating revenues due to the vehicle down time over the life of the vehicle.
In addition to anti-skid systems, systems have been suggested which allow the operator to manually adjust or select braking ratios of the various wheels. In general, these systems involve a variable restriction placed in appropriate pressure conduits or adjustable biasing of control valves. The operator is then provided with an adjustment, in some cases poppet-type valves, to control the amount of restriction or biasing. It should be readily apparent that the changes in braking ratios is not accomplished automatically but requires action by the vehicle operator.
Operator adjustable braking ratios are now precluded for many vehicles by Federal statute or regulations. These regulations mandate that a vehicle be able to stop within specified distances in a controlled manner and do so without operator intervention to adjust for vehicle operating conditions.
Other systems have been suggested which employ ratio valves to modify the flow of pressure to the front or rear brake actuators under various vehicle operating conditions. Such a system would provide a means for controlling the operation of the ratio valve so that its effect on brake pressure flow could be changed in response to vehicle operating modes. A problem associated with a brake system employing this type of ratio control, is that the ratio valve will be operated to reduce the flow to one or more brake actuators in all braking applications. These systems would reduce the flow of pressure to the associated brake actuators even if conditions warranted maximum brake application. Full treadle depression would not cause full system source pressure to be applied to the brake actuators. The pressure applied would in fact be source pressure reduced by the ratio valve. Thus, full braking would not be available on all vehicle wheels.
Several prior art proposals have also been suggested which do not modify braking ratios but only modify the "feel" of the braking system when operating without a trailer. This brake feel is in actuality the sensitivity of the operator controlled treadle valve. The problem addressed by this proposal is the over-braking that may occur when the tractor is operated without a trailer. This over-braking is caused by the excessive depression of the treadle valve of the operator who is conditioned to braking the tractor-trailer combination. Less depression of the treadle valve is necessary to stop the tractor alone than is needed to stop the tractor-trailer combination. An operator accustomed to the effort necessary to brake a tractor-trailer vehicle combination may find himself over-braking the tractor when operating without a trailer.
The proposed system discloses a method by which pressure going to the trailer is monitored. This pressure is used to control a reaction piston which acts against the operator foot force being applied to the treadle valve. The absence of a trailer, results in atmospheric pressure being present in the conduit conveying fluid pressure to the trailer. This proposed system utilizes this absence of fluid pressure to effect a maximum reaction force against the operator applied force. The resulting increased effort necessary to depress the brake treadle valve, is intended to reduce the tendency of the operator to over-depress the brake treadle valve.
Vehicle control and brake system performance during braking is also related to the sequence of brake application. In light brake applications, the minimum pressure or "crack-open pressure", required to initially open brake system control valves will usually dictate the order of brake application. In ordinary brake system control valves, valve hysteresis plays an important role in determining the crack-open pressure for the particular valve. If the brake system design does not properly account for differences in valve crack-open pressures, unbalanced braking may result especially in light brake applications. If substantial differences exist in crack-open pressures, it is possible that under certain braking situations, (usually very light brake applications) some of the brakes may not engage at all.